fix(welcome-msg): conditional display of Container Operations and Pre-commit sections

[myakove]

Problem

The welcome message displays features that may not be configured for the repository:

1. Container Operations Section - Always Shown

• Location: webhook_server/libs/handlers/pull_request_handler.py:281-283
• Current behavior: The "Container Operations" section with /build-and-push-container is unconditionally displayed
• Expected behavior: Only show when container is configured for the repository

2. Pre-commit Checks Line - Always Shown

• Location: webhook_server/libs/handlers/pull_request_handler.py:255
• Current behavior: The "Pre-commit Checks" line is unconditionally displayed in automatic actions
• Expected behavior: Only show when pre-commit is enabled in config

Current Code (Problematic)

# Line 255 - always shown
* **Pre-commit Checks**: [pre-commit](https://pre-commit.ci/) runs automatically if `.pre-commit-config.yaml` exists

# Lines 281-283 - always shown
#### Container Operations
* `/build-and-push-container` - Build and push container image (tagged with PR number)
  * Supports additional build arguments: `/build-and-push-container --build-arg KEY=value`

Reference: Correct Pattern

The _prepare_retest_welcome_comment property (lines 345-367) correctly implements conditional logic:

if self.github_webhook.build_and_push_container:
    retest_msg += f" * `/retest {BUILD_CONTAINER_STR}` - Rebuild and test container image\n"

if self.github_webhook.pre_commit:
    retest_msg += f" * `/retest {PRE_COMMIT_STR}` - Run pre-commit hooks and checks\n"

Suggested Fix

Apply the same conditional pattern to:

1. Wrap Container Operations section with if self.github_webhook.build_and_push_container:
2. Wrap Pre-commit Checks line with if self.github_webhook.pre_commit:

Impact

Users of repositories without container/pre-commit configuration see commands that don't work for their repository, causing confusion.